Proton maser antenna tuning means

ABSTRACT

Tuning means capable of extending the low frequency sensitivity of a proton maser antenna to higher frequencies are realized by magnetically chopping the effective permeability of the maser antenna magnet at the local oscillator frequency. This effectively chops the received signal at the local oscillator rate and translates it to the fixed system frequency fIF. The system electronics are tuned to receive only the maser sidebands corresponding to fIF, and tuning is accomplished by varying the local oscillator frequency.

Hittite tates atent 1 1 Webster et al. 14 1 Apr. 3, 1973 [541 PROTUNMASER ANTENNA TUNING 3,195,061 7/1965 Jeffriesetal. ..330/4 MEANS OTHERPUBLICATIONS [75] Inventors: Jonathan C. Webster; Earl L. Sloan, A G s t1 st d d I f In bth fkd FM 4 ansene a., uyan nve 1 ga1ono a 5 ei g figgg Proton Maser Antenna, 4/65, T.R.N. RADC-TR-64- 567. many [73]Assignee: The United States of America as Primary Examiner-Benjamin A.Borchelt represented by the Secretary of the Assistant Examiner-N.Moskowitz Air Force Att0rneyI-larry A. Herbert, Jr. and Willard R. 22Filed: Sept. 3, 1971 Matthews [21] Appl. No.: 177,751 [57] ABSTRACTTuning means capable of extending the low frequency 52 US. 01. ..330/4,332/75, 325/485 Sensitivity of a proton maeer antenna to higher 51 Int.Cl ..H01s 1 02 frequencies are realized by megnetieelly ehepping the[58] Field of Search 330/45 4; 331/94. effective permeability of themaser antenna magnet at 325/485 the local oscillator frequency. Thiseffectively chops I the received signal at the local oscillator rate and[56] References Cited' translates it to the fixed system frequency fur.The system electronics are tuned-to receive only the maser UNITED STATESPATENTS sidebands corresponding to f and tuning is accomplished byvarying the local oscillator frequency. 3,564,258 2/1971 Fringold 330/43,333,101 7/1967 Bell ..250/83.3 3 Claims, 5 Drawing Figures M J0:011:40: nfzz fzan amaz i flUD/d nruim unlmxaHammalmaHzar/n/H] Aura/152t (I, #1r/wmlumfli k/xx/Mw;I- i q BACKGROUND OF THE INVENTION Protonmaser antennas initially were developed to improve the reception of verylow frequency electromagnetic waves. The loop antenna has been thetraditional apparatus employed in the reception of such signals;however, certain difficulties become apparent as the wavelength of thesignals becomes very long.

The rapid deterioration of conventional loop performance as the carrierfrequency is reduced results from the unfavorable ratio of the antennasize to the signal wavelength. If ideal reception were to be achieved,the loop should extract all the energy passing within a radius of z)t/S. In practice, the loop may have an effective aperture not muchlarger than its own physical dimensions. When this picture is couched inelectrical terms one says that the radiation resistance of the antennais negligible compared to its internal ohmic losses. Hence, it is notefficiently coupled to the external radiation field. In particular, thefield strength must exceed a minimum value so that the fraction ofsignal power intercepted is greater than the internally generated kTBnoise power.

Very simply, since the induced Emf is proportional to the rate of changeof the flux through the loop, the detected Emf will be inverselyproportional to the wavelength. Therefore, for a fixed field strength,there is a minimum detectable frequency below which the signal will bemasked by internal thermal noise.

The proton maser antenna is a device which has been developed toovercome these problems. Basically, such a device consists of a protonmaser which is frequency modulated by the direct interaction of theelectromagnetic field with dynamically polarized protons. The resultingsidebands are processed and detected to reconstitute the originalsignals.

By appropriate frequency multiplication and FM detection, even very lowindices of modulation of the maser can be detected. The sideband power(for a given signal strength) is directly proportional to the maserpower. Hence the useful signal power in the sidebands can greatly exceedthat which the loop might have extracted from the passing wavefront.Since both sensors are competing against the same fundamental kTBJohnson noise, the potentialities of the proton maser antenna scheme canbe appreciated. Furthermore, the nature of the FM sideband power is suchthat as the signal carrier frequency is reduced, the proton maserantenna detection capability is increased.

The Proton Maser Antenna is described in detail in the National Company,Inc. report designated Technical Report No. RADC-TR-64-567 entitledStudy and Investigation Of A Proton Maser Antenna by Alexander Ganssenet a1. dated April 1965.

Although the Proton Maser Antenna has proved effective, there hasremained low frequency tuning problems to be solved. In particular, itwould be very desirable to extend the improved low frequency sensitivityof such a device to higher frequencies.

This problem has been approached by tuning the system electronics topick up the desired sideband of the maser. This method, however, has thedisadvantage that the modulation index decreases with increasing carrierfrequency, thereby giving decreased sensitivity at the higherfrequencies.

Although it would be possible to achieve low frequency tuning byperiodically chopping the effective permeability of the maser magnet,there are no currently available chopping devices capable of effectingthe required chopping function. For instance, state of the artmechanical choppers require extremely high rotation rates for highcarrier frequencies and saturating schemes require huge amounts of powerin order to saturate coils at high frequencies. The present inventioncomprehends a proton maser tuning device that utilizes the periodicchopping technique and that provides new and improved circuits foraccomplishing the chopping function.

SUMMARY OF THE INVENTION The present invention achieves tuning of theproton maser antenna by a heterodyning scheme which comprehendsmu-switching the maser magnet. The maser magnet of a conventional protonmaser antenna is replaced by a magnet having flux concentrating membersprotruding from opposite poles. Series connected eddy current coils arewound around the flux concentrating members and are periodically shortedout by switching means. The switching means is operated at the systemlocal oscillator frequency. This chops the effective permeability of themagnet at system frequency. Tuning is achieved by varying the localoscillator frequency. The system electronics are tuned to receive onlythe maser sidebands corresponding to the system frequency. A uniquediode switch is used which provides both high frequency switchingcapability and low on resistance in the eddy current coil circuit.

It is a principal object of the invention to provide a new and improvedproton maser antenna that can be tuned over a wider frequency range thansimilar currently available antennas.

It is another object of the invention to provide a proton maser antennathat is tuned by means of periodically chopping the effectivepermeability of the maser antenna magnet.

It is another object of the invention to provide a proton maser antennaof the type described having low resistance high frequency diodeswitching means.

These, together with other objects, advantages and features of theinvention, will become more apparent from the following detaileddescription taken in conjunction with the illustrative embodiments inthe accompanying drawings.

DESCRIPTION OF THE DRAWING FIG. 1 is a schematic, partly in the form ofa block diagram, illustrating a proton maser antenna in combination withthe tuning means of the present invention; and

FIG. 2 illustrates the various waveforms associated with the magneticheterodyning effects of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The basic proton maser antennacomprehended herein is illustrated in block diagram form in FIG. 1. Suchan antenna is described in detail in the National Company reportincorporated herein by references.

The tuning means of the invention comprises maser magnet 4, fluxconcentrators 5 and 6, eddy current coils 7 and 8, oscillator frequencycontrol 16 and switching means consisting of diodes 9 and 10 and audioamplifier 11.

The heterodyne method of the invention which is analogous to asuperheterodyne receiver permits the reception of higher frequencieswith a sensitivity comparable to that attained for low frequencyreception. This is accomplished by magnetically chopping the receivedsignal at the local oscillator rate, thereby translating it to the fixedsystem frequency flp. The electronics are then tuned to receive only themaser sidebands corresponding to f and tuning is accomplished by varyingthe local oscillator frequency.

The magnetic field at the sample resulting from the received signal ischopped by changing the effective permeability of the magnetic structureat the local oscillator rate. The following is a mathematicaldemonstration that this technique produces the desired results amagnetic field varying with the frequency f at the sample. The receivedsignal may be expressed as:

Let it be assumed that the effective permeability of the magnetstructure, u fl), is being varied so that the fundamental component isat the frequency f and that the form of p fl) If it is also assumed thatf and f are chosen so that there will be no interference from componentsat frequencies 2f f,, 3f -f,, etc., then u flt) can be written in theform:

Ma) =m [(10% cos ir H,,(t)=H, [c l-a cos t)] cos t) f L) cos giving:

The term oscillating at the sum frequency is not of interest since itwill not be detected by the electronics,

therefore:

H,,(t) =a H cos 10+? H cost) where it is assumed that f has beenadjusted so that f, fL0 f This is exactly the desired result. Theassociated waveforms for an idealized case in which a is switched on andoff are shown in FIGS. 20 through 2d. Having reference to these figures,waveform 12 is the received signal, waveform 13 illustrates theeffective permeability, waveform 14 illustrates the magnetic field atthe sample and waveform 15 illustrates the magnetic field observed witha long time constant.

Since the effective permeability cannot be less than zero and since itcannot exceed the maximum CW value (u fl it can be concluded that:

o+ l (#21!) max where any non-sinusoidal characteristics of u fl) u ahave been neglected. The practical problem then is reduced to making a,a as small as possible. This is accomplished by the eddy currentswitching means of the present invention.

The basic concept of the invention utilized to accomplish periodicswitch of the effective permeability of the maser magnet is that of eddycurrent flux-cancelling. Eddy current coils 7 and 8 are wound around theflux concentrators 5 and 6 as shown in FIG. 1. The operation of thismethod is based on the exclusion of time varying flux from a shortedturn. That is, the emf induced in a shorted coil gives rise to an eddycurrent which, in turn, produces a flux which tends to cancel theapplied flux. The resistance around the loop of the coil plus its short"must be kept at a minimum for this method to be effective.

It is therefore necessary to use a switch that can operate at up to10,000 Hz and yet have extremely low on resistance. This is accomplishedby means of switching diodes 9 and 10 and audio amplifier 11. The audioamplifier and hence the switching frequency are responsive to the systemlocal oscillator. The diodes are chosen for minimum potential developedbetween points A and B.

It will be understood that various changes in the detailed materials andarrangements of parts which have been described and illustrated in orderto explain the nature of the invention may be made by those skilled inthe art within the principle and scope of the invention as expressed inthe appended claims. For example, various switching schemes and othermechanics for chopping the effective perambility of the maser magnetcould be used.

What is claimed is:

1. The combination of a proton maser antenna having a local oscillatorand magnet means and tuning means, said tuning means comprising:

means for varying the frequency of said local oscillator, and

means for chopping the effective permeability of said magnet means atthe local oscillator frequency.

2. The combination of a proton maser amplifier and tuning means definedin claim 1 wherein said magnet means includes flux concentration means.

3. The combination of a proton maser amplifier and tuning means definedin claim 2 wherein said means for chopping the effective permeability ofthe magnet means comprises eddy current coil means operably connected tosaid flux concentration means and means for shorting said coil means atthe local oscillator frequency.

1. The combination of a proton maser antenna having a local oscillatorand magnet means and tuning means, said tuning means comprising: meansfor varying the frequency of said local oscillator, and means forchopping the effective permeability of said magnet means at the localoscillator frequency.
 2. The combination of a proton maser amplifier andtuning means defined in claim 1 wherein said magnet means includes fluxconcentration means.
 3. The combination of a proton maser amplifier andtuning means defined in claim 2 wherein said means for chopping theeffective permeability of the magnet means comprises eddy current coilmeans operably connected to said flux concentration means and means forshorting said coil means at the local oscillator frequency.